When the horizontal output stage of a video display fails, the defective components are often replaced without first testing for defects in the horizontal drive stage. If the horizontal drive stage is in fact the cause of the failure of horizontal output stage components, the substituted components may operate for a short interval of time, allowing the display to be placed back in service, only to fail again. Multifrequency video displays are adapted to operate through a range of horizontal frequencies, and the horizontal drive stage must produce a base current to the horizontal output transistor which is suitable for the current required for each frequency at which the display operates. Displays with a wide horizontal operating range frequently have horizontal drive stages which are capable of switching between two or more levels of base current to the horizontal output transistor (H.O.T.) so that it is lower at lower frequencies and higher at high frequencies. A lower base current is needed for lower frequencies because the H.O.T. conducts for longer periods of time at the lower frequencies thereby allowing a significantly higher collector current than would occur if the same collector current were applied at higher frequencies. Two methods are commonly employed in the driver stage to switch the current level to the base of the H.O.T. One method is to change the level of the primary current in the driver transformer by changing the primary resistor which is in series with the windings of the transformer. The second method is to switch a resistor in series with the output for the driver circuit and the base of the H.O.T.
If the base current to the H.O.T. drops below its designed operating range, the H.O.T. will not provide the collector current which is normally required for proper operation of the display. The symptoms may not be detectable if the current is reduced only slightly, but more significant reductions will cause the collector current to become limited, thereby limiting the horizontal deflection. As the base current is reduced, the efficiency of the H.O.T. is correspondingly reduced, and this will cause the H.O.T. to overheat and prematurely fail.
Intermittents, or fluctuations in the driver stage output current, can also cause failure of the H.O.T. This is because intermittents cause the H.O.T. to be turned on and off at inappropriate times thereby reducing the efficiency of the H.O.T. and causing it to heat excessively.
If service personnel merely replace a defective H.O.T. and return the video display to service without first testing the driver stage output to determine if it is providing the current required for each frequency of the display and that it is not subject to intermittence, the repaired video display may again fail shortly after it is returned to service. Presently, there is no method or apparatus for testing the output of the horizontal driver stage, and it would be desirable to provide a method and apparatus for undertaking such a test. Since the horizontal output stage operates at high voltages, it would also be desirable that the instrument analyzes the equipment under test without energizing the horizontal output stage of the display to reduce the danger of shock to service personnel or damage to test equipment.